Earth boring bits used for drilling holes in the earth are typically classified into two types: drag bits which have no moving parts and shear the formation (e.g. polycrystalline diamond compact (PDC) bits, diamond impregnated bits, etc.) and rotary cone bits which have one or more generally conic roller cones rotatably mounted on the bit body. The roller cones have cutting teeth and/or inserts extending therefrom and rotation of the bit body rotates the cones so that the cutting teeth and/or inserts crush and gouge the formation.
Both of these types of bits use nozzles mounted on the bit body to direct drilling fluid coming down the drill string to sweep the bottom of the borehole and carry cuttings back up the hole on the outside of the drill string. This fluid flow, or "bit hydraulics", serves three primary purposes: cutting removal, relief of chip hold down pressure, and, in the case of rotary cone bits, cleaning of the cones. The location and type of the nozzles used can greatly impact these purposes.
Location of the nozzles relative to the borehole bottom is especially relevant to rotary cone bits versus drag bits. Because the face of the drag bit body is directly against the formation, the nozzles in a drag bit are readily located near the borehole bottom by mounting of a nozzle in a receptacle in the bit body. In contrast, the bit body of a rotary cone bit is disposed above the bottom of the formation by the rotary cones and thus fluid exiting from a nozzle recessed or flush with the bit body must travel a significant distance before impinging at or near the borehole bottom. Moving the nozzle exit closer to the hole bottom can generally improve chip removal by increasing the bottom hole energy and by improving the ability of the fluid to relieve chip hold-down pressures.
One way the exit orifice of nozzles in rotary cone bits have been moved closer to the borehole bottom is by using steel tubes that extend from the bit body with a wear-resistant nozzle mounted in the end of the tube. These extended nozzle tubes have the advantage of being able to closely locate the exit orifice of the nozzle close to the borehole bottom; however, the extended tubes are susceptible to breaking. A tube breaking off of the bit effectively ends the run of that particular bit and may require a costly down hole fishing (retrieving) operation to remove the tube from the bottom of the borehole.
Another way that the exit orifice has been moved closer to the borehole bottom is by the use of "mini-extended" nozzles. Conventional nozzles are generally flush or recessed from the outer surface of the receptacle in the bit body in which they are mounted. Mini-extended nozzles have a portion which extends beyond the receptacle in which it is mounted but still are retained by conventional nozzle retention means. With reference to FIG. 1, a conventional mini-extended nozzle 10 is shown mounted in receptacle 12 defined in bit body assembly 14 with fluid bore 15. Nozzle 10 defines passage 16 for the direction of drilling fluid through the nozzle. Receptacle 12 conventionally has a standard inner diameter for a given size bit. Retainer 18 threads into receptacle 12 at threaded connection 24 and retains nozzle 10 in receptacle 12 by capturing shoulder 20 of nozzle 10 by ledge 22 extending radially inward from retainer 18. Nozzle 10 seats on shoulder 26 in receptacle 12. Seal 28 seals between the outer surface of nozzle 10 and the inside of receptacle 12. Nozzle 10 is referred to as a "mini-extended" nozzle due to the fact that the nozzle has portion 11 extending beyond receptacle 12. The outer diameter of portion 11 is smaller than the outer diameter of base portion 13 of nozzle 10 in order to extend beyond ledge 22 of retainer 18. The advantage of mini-extended nozzles is their relative durability and ruggedness compared to extended tubes; however, a mini-extended nozzle does not locate the nozzle orifice as close to the borehole bottom as an extended tube.
U.S. Pat. No. 5,669,459 discloses a retention body for holding a mini-extended nozzle closer to the borehole bottom. This design has the advantage of better protecting the mini-extended nozzle during operation by extending a mild steel retention body along the portion of the nozzle that extends beyond the body of the bit. By better protecting the nozzle, the orifice of the nozzle can be moved closer to the borehole bottom compared to a mini-extended nozzle mounted in a conventional receptacle while at the same time avoiding the potential breakage problems associated with extended tubes.
Thus for a rotary cone bit, the mini-extended nozzle can be used in a conventional receptacle for some extension, with a retention body of the '459 patent for additional extension, or with an extended tube for even more extension but with risk of tube breakage.
In addition to location of the nozzle in the axial direction (i.e., distance from borehole bottom), the type of nozzle used impacts the goals of chip removal, relief of chip hold down pressure, and cone cleaning. More specifically, the nozzle passageway and orifice can effect bit hydraulics. U.S. Pat. No. 5,494,124 (as well as related patents U.S. Pat. Nos. 5,632,349; and 5,653,298) discloses a type of nozzle with a passageway and orifice design that is purported to provide advantages over other nozzles when used in an earth boring bit. FIGS. 1, 3, and 5 of the '124 patent show the shaped orifices (slot 16, 46, and 76, respectively) while FIGS. 2, 4, and 6 of the '124 patent show the corresponding internal passage 20, 50, 80, respectively.
With reference to FIG. 2, an embodiment of nozzle 10' of the type disclosed in the '124 patent is shown in receptacle 12 with retainer 18 capturing end 21 of nozzle 10'. Nozzle 10' is recessed from the opening of receptacle 12. Passage 16' has transition zone 29 that transitions from passage 16' to orifice 31. The '124 patent teaches particular shapes of transition zone 29 and orifice 31 to achieve the desired fluid characteristics for the nozzle.
One disadvantage of the nozzle of the '124 patent is that its internal passage 16' must be much larger than that of a conventional nozzle to allow sufficient room for the desired short transition zone 29 with its high rate of inward taper to orifice 31, especially for larger sized nozzle orifices. The standard receptacle 12 in a bit together with the retention means used to hold the nozzle in the receptacle limits the maximum outer envelope of the nozzle, and this together with the minimum acceptable wall thickness of the nozzle limits the maximum size of internal passage 16' of the nozzle. Thus, for a given receptacle 12, the maximum nozzle orifice size achievable by the '124 nozzle will be appreciably less than that of a conventional nozzle. This is a disadvantage because standard drilling practices often require larger nozzle orifices to reduce the pressure drop across the bit. The inability to accommodate larger nozzle orifices makes the nozzles of the '124 patent less versatile and unable to be used in certain drilling applications that may require a pressure drop that is less than that available with the largest '124 nozzle for the particular receptacle in the bit.
This disadvantage of the '124 nozzle is compounded when it is desired to take advantage of the mini-extended nozzle concept by extending the end of the nozzle beyond the receptacle in which it is mounted. Retainer 18 used with mini-extended nozzle 10 in FIG. 1 requires a reduced outer diameter of extended portion 11. This reduced diameter even more severely restricts the maximum size of internal passage 16' of the '124 type nozzle of FIG. 2 thus further reducing the maximum nozzle exit orifice size possible relative to a mini-extended nozzle with a conventional internal passage.
Furthermore, the nozzle of the '124 patent relies in part on a relatively short transition zone 29 to taper from passageway 16' to orifice 31. Passageway 16' only slightly tapers radially inward from interior end 19 to transition zone 29 and thus maintains a relatively large inner diameter compared to passageway 16 in FIG. 1. Transferring passageway 16' to a mini-extended nozzle of FIG. 1 can be seen by the dashed line in FIG. 1 which represents extended passageway 16" for a nozzle of the type of the '124 patent. As can be seen the inner diameter of passageway 16" is larger than the outer diameter of extended portion 11 at a point indicated at 17. Thus, such an extension is not possible with retainer 18 of FIG. 1.
While nozzles of the type of the '124 patent have been used with drag bits as shown in FIG. 2, they are not directly translatable to a rotary cone bit without the disadvantages discussed above. Therefore, a need exists for a nozzle and retainer assembly that allows for an increase in the size of the internal passage of a mini-extended nozzle so that the teachings of the '124 patent can be used in a mini-extended design for a range of nozzle orifice sizes comparable to that of conventional mini-extended nozzles.
One teaching of the '124 is the generation of lower than hydrostatic pressure zones on the hole bottom. In drilling applications, fluid is transmitted to the hole bottom via a drill string to remove cuttings from the hole bottom and transport them back to the surface through the annular space between the drill string and the hole wall. Weighting materials are typically added to the drilling fluid to ensure the bore hole pressure is greater than that of the pore pressure to ensure the integrity of the bore hole. If the fluid is under-weighted, causing the bore pressure to be less than the pore pressure of the surrounding formation, the hole can cave in and stick the drill string in the hole which causes costly hole deviations. However, if the hole pressure is too high, rock bit penetration rates are significantly reduced since the chips generated by the cutters tend to be held in the formation by the pressure differential across the hole surfaces. The '124 nozzles are intended to generate localized low pressure zones on the hole bottom which allows cuttings to lift from the hole bottom in these localized zones in the presence of global overburden pressures. To generate the localized low pressure zones, the '124 nozzles are intended to generate lobes of flow which move the fluid radially outboard from the centerline of the nozzle. Because the flow from the '124 nozzles is not axisymmetric like that of nozzle 10 in FIG. 1, a need exists to optimize the rotational position of the nozzles relative to the cones of a rotary cone bit.
Additionally, nozzles may have passages and/or asymmetric orifices that direct the fluid at an angle. As fluid flows through an angled passage, it will impart a rotational force on the nozzle. Such nozzles must be able to be readily located at a desired rotational orientation and/or locked against rotational forces from fluid flow through the bit. Thus a need exists for a nozzle and retainer assembly that allows for an increase in the size of the internal passageway of a mini-extended nozzle and provide for rotational location and/or locking of the nozzle relative to the bit body.